1. Field of the Invention
The present invention relates to devices that hold and center a workpiece about an axis of rotation of a lathe.
2. Description of the Prior Art
Wood turning lathes typically have a driven tool center at the headstock end, and either a tool center that freely rotates (live center) or a dead center that does not rotate at the tailstock end. The driven center typically has a plurality of spurs or projections, and in some cases, a conical tip to engage the workpiece. The projections and the conical tip are pounded or forced into one end of the workpiece. The purpose of the conical tip is to center the workpiece while the projections provide a mechanical connection to transmit the driving force from the driven tool center to the workpiece.
Dead tool centers normally have an inverted conical ring which engages the workpiece, although dead tool centers have been known to have a central conical tip. Live tool centers, on the other hand, rotate on bearings and have a protruding conical tip which engages the workpiece. Dead tool centers, since they do not rotate, generate heat from frictional engagement between the tip and the workpiece creating a hole in the workpiece. Lubricants such as bee's wax have been used to reduce friction, but the results have not been satisfactory. Live tool centers, having a sharp conical point, tend to split the wood workpiece when axial pressure is applied to hold the wood workpiece between the tool centers of the lathe.
A further problem with the prior art relates to maintaining the same axial center position when the workpiece is removed and reinserted back into the lathe. The projections of the prior art drive centers generally do not re-seat in the same holes in the wood workpiece that were previously formed, thereby causing the wood workpiece to be off the previous center resulting in difficulties in the wood turning operation.
In certain other operations, it is desirable to remove the wood workpiece and reinsert it in the lathe end-for-end. This operation causes even greater difficulties in attempting to re-center the wood workpiece about the previous center of rotation. Consequently when using the prior art devices, most turning operations are completed before removing the wood workpiece from the lathe.
Some of the prior art devices are described in the following prior art patents. Schwartz U.S. Pat. No. 2,346,192 shows a tool center with an end having a step configuration and spurs projecting therefrom. The tool center of the Schwartz patent, however, is limited to workpieces which have two halves temporarily united for the turning operation on a lathe.
Barnes U.S. Pat. No. 307,901; the Cook et al U.S. Pat. No. 2,879,816; and the Schmidt U.S. Pat. No. 4,141,397 show chuck assemblies for holding workpieces within a lathe. None of the chuck assemblies of the immediately above-cited patents provides for reinsertion and centering of the workpiece along the same axis of rotation.
The Christensen U.S. Pat. No. 2,876,807; the Williams U.S. Pat. No. 2,909,955; and the Colledge U.S. Pat. No. 2,923,238 describe tool centers which have a conical tip and projections of different configurations which engage the workpiece. Although the devices in the immediately-above cited patents provide for some centering of the workpiece, they do not solve the previously-mentioned problems of splitting the workpieces, frictionally burning the ends of the workpieces, and when reinserting the workpiece, having the workpiece rotating about a different axis than when doing the previous work.